The traditional computer navigational device uses an interior rolling ball to track its motion across a surface. The optical mouse was developed in response to problems with the common roller-ball mouse. An optical mouse does not have moving parts, but instead contains a sensor array arranged to take successive images of the surface supporting the mouse. Each image of the surface is compared to the proceeding image, and from this comparison a system can determine how far the mouse has moved. This system may not always operate effectively if the imaged surface is smooth.
Such devices use the constructive and destructive interference of the electromagnetic waves to form patterns of high and low intensity areas called speckle patterns. The contrast and size of these patterns may take various forms dependent upon, among other things, the coherency or lack of coherency of the electromagnetic waves as well as the arrangement of the device.